Electromagnetics and Photonics Commons^™

Design And Development Of A Low-Cost High Range Resolution X-Band Radar, Paul C. Cantu

Department of Electrical and Computer Engineering: Dissertations, Theses, and Student Research

Synthetic Aperture Radar (SAR) is one of the main tools for microwave remote sensing because of its multi-dimensional high resolution characteristics and the capability to operate in nearly all weather conditions, day and night. The University of Nebraska-Lincoln (UNL) initiated the design and development of a low-cost airborne SAR in January 2001 to support our Airborne Remote Sensing Program. The objectives of this project are separated into various evolutionary stages. This thesis will focus on the initial phase of design and construction of an X-band high range resolution radar (HRR) using basic RF /microwave and digital components. The following stages …

Photonic Crystals: Modeling And Simulation, Shangping Guo

Electrical & Computer Engineering Theses & Dissertations

Photonic crystals are periodic electromagnetic media in optical wavelength scale. They possess photonic band gaps (PBGs) that inhibit the existence of light within the crystals in certain wavelength range. Such band gaps produce many interesting optical phenomena. In this dissertation, the frequency (plane wave method, PWM) and time domain (finite difference time domain method, FDTD) methods are developed for their modeling and simulation.

The theory and algorithm of plane wave method are studied in detail and implemented in a unique and efficient approach. PWM is used to obtain the gap and mode information of ideal and defective photonic crystals. Several …

Wide Angle Achromatic Prism Beam Steering For Infrared Countermeasures Applications, Bradley D. Duncan, Philip J. Bos, Vassili Sergan

Electro-Optics and Photonics Faculty Publications

The design and analysis of achromatic doublet prisms for use in laser beam steering is presented. The geometric relationships describing the maximum steering angle are given, as are discussions of first- and second-order dispersion reduction. Infrared (IR) material alternatives and optimum IR material characteristics for wide-angle achromatic prism beam steering are also investigated. Sixteen materials in 120 different combinations have been examined to date. For midwave IR applications it is shown that the minimum dispersion currently achievable across the full 2 to 5 μm spectrum is 1.7816 mrad at an average maximum steering angle of 45 deg. This is accomplished …

Algan/Gan Heterostructure Field-Effect Transistors On Single-Crystal Bulk Aln, X. Hu, J. Deng, N. Pala, R. Gaska, M. S. Shur, C. Q. Chen, J. Yang, Grigory Simin, M. A. Khan, J. C. Rojo, L. J. Schowalter

Faculty Publications

We report on the performance of AlGaN/GaN/AlN heterostructurefield-effect transistors(HFETs) grown over slightly-off c-axis, single-crystal, bulk AlN substrates. Dc and rf characteristics of these devices were comparable to HFETs grown on semi-insulating SiC. The obtained results demonstrate that bulk AlN substrates are suitable for fabricating high-power microwave AlGaN/GaN transistors.

Index-Matched Boundary Techniques For The Elimination Of Acoustical Resonances, Jack H. Parker, Bradley D. Duncan

Electro-Optics and Photonics Faculty Publications

We extend the principle of optical index of refraction to apply the concept of acoustical index for transverse acoustical wave propagation in strings. The relationship between acoustical index and mass density of the acoustic material is developed. With this theoretical link established, classic index-matching techniques are explored at acoustical boundaries. Proper selection of boundary interface segments leads to the elimination of resonant vibrationalmodes that occur in rigidly supported strings, while maintaining the nonresonant vibration response.

Time-Resolved Electroluminescence Of Algan-Based Light-Emitting Diodes With Emission At 285 Nm, M. Shatalov, A. Chitnis, V. Mandavilli, R. Pachipulusu, J. P. Zhang, V. Adivarahan, S. Wu, Grigory Simin, M. Asif Khan, G. Tamulaitis, A. Sereika, I. Yilmaz, M. S. Shur, R. Gaska

Faculty Publications

We present a study on the time evolution of the electroluminescence(EL)spectra of AlGaN-based deep ultraviolet light-emitting diodes(LEDs) under pulsed current pumping. The ELspectra peaks at 285 nm and 330 nm are found to result from recombination involving band-to-band and free carriers to deep acceptor level transitions. The 330 nm long-wavelength transitions to deep acceptor levels in the p-AlGaN layer as well as the nonradiative processes significantly influence the LED internal quantum efficiency.

Digital Image Processing, Russell C. Hardie, Majeed M. Hayat

Electrical and Computer Engineering Faculty Publications

In recent years, digital images and digital image processing have become part of everyday life. This growth has been primarily fueled by advances in digital computers and the advent and growth of the Internet. Furthermore, commercially available digital cameras, scanners, and other equipment for acquiring, storing, and displaying digital imagery have become very inexpensive and increasingly powerful. An excellent treatment of digital images and digital image processing can be found in Ref. [1]. A digital image is simply a two-dimensional array of finite-precision numerical values called picture elements (or pixels). Thus a digital image is a spatially discrete (or discrete-space) …

Numerical Techniques For Excitation And Analysis Of Defect Modes In Photonic Crystals, Shangping Guo, Sacharia Albin

Electrical & Computer Engineering Faculty Publications

Two numerical techniques for analysis of defect modes in photonic crystals are presented. Based on the finite-difference time-domain method (FDTD), we use plane wave incidences and point sources for excitation and analysis. Using a total-field/scattered-field scheme, an ideal plane wave incident at different angles is implemented; defect modes are selectively excited and mode symmetries are probed. All modes can be excited by an incident plane wave along a non-symmetric direction of the crystal. Degenerate modes can also be differentiated using this method. A proper arrangement of point sources with positive and negative amplitudes in the cavity flexibly excites any chosen …

Milliwatt Power Deep Ultraviolet Light-Emitting Diodes Over Sapphire With Emission At 278 Nm, J. P. Zhang, A. Chitnis, V. Adivarahan, S. Wu, V. Mandavilli, R. Pachipulusu, M. Shatalov, Grigory Simin, J. W. Yang, M. Asif Khan

Faculty Publications

We report on AlGaN multiple-quantum-well (MQW)-based deep ultraviolet light-emitting diodes over sapphire with peak emission at 278 nm. A new buffer layer growth process was used to reduce the number of defects and hence the nonradiative recombination. The improved material quality and carrier confinement resulted in pulsed powers as high as 3 mW at 278 nm and a significantly reduced deep-level-assisted long-wavelength emission.

Chaotic Free-Space Laser Communication Over Turbulent Channel, N. F. Rulkov, Mikhail Vorontsov, L. Illing

Electro-Optics and Photonics Faculty Publications

The dynamics of errors caused by atmospheric turbulence in a self-synchronizing chaos-based communication system that stably transmits information over a ∼5  km free-space laser link is studied experimentally. Binary information is transmitted using a chaotic sequence of short-term pulses as a carrier. The information signal slightly shifts the chaotic time position of each pulse depending on the information bit. We report the results of an experimental analysis of the atmospheric turbulence in the channel and the impact of turbulence on the bit-error-rate performance of this chaos-based communication system.

Polarization Effects In Photoluminescence Of C- And M-Plane Gan/Algan Multiple Quantum Wells, E. Kuokstis, C. Q. Chen, M. E. Gaevski, W. H. Sun, J. W. Yang, Grigory Simin, M. Asif Khan, H. P. Maruska, D. W. Hill, M. C. Chou, J. J. Gallagher, B. Chai

Faculty Publications

Polarizationeffects have been studied in GaN/AlGaN multiple quantum wells(MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence(PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [11̄00]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs.Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. …

Electromagnetic Scattering From A Gap In A Magneto-Dielectric Coating On An Infinite Ground Plane, George R. Simpson

Theses and Dissertations

The electromagnetic scattering from a gap in a magneto-dielectric coating on an infinite ground plane is analyzed. In this context, the gap forms a break only in the magneto-dielectric slab coating while the ground plane is continuous and unbroken. Volume equivalence is used to convert the gap region to one containing unknown volumetric equivalent electric and magnetic currents. The equivalent problem then is one of these currents radiating in the presence of an unbroken grounded magneto-dielectric slab. A Green's function for this geometry is developed consisting of two terms: a direct coupling term and correction term to account for the …

Gan Homoepitaxy On Freestanding (11̄00) Oriented Gan Substrates, C. Q. Chen, M. E. Gaevski, W. H. Sun, E. Kuokstis, J. P. Zhang, R. S. Q. Fareed, H. M. Wang, J. W. Yang, Grigory Simin, M. A. Khan, Herbert-Paul Maruska, David W. Hill, Mitch M. C. Chou, Bruce Chai

Faculty Publications

We report homoepitaxialGaNgrowth on freestanding (11̄00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition.Scanning electron microscopy,atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (11̄00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (11̄00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, …

Low-Temperature Operation Of Alfan Single-Quantum-Well Light-Emitting Diodes With Deep Ultraviolet Emission At 285 Nm, A. Chitnis, R. Pachipulusu, V. Mandavilli, M. Shatalov, E. Kuokstis, J. P. Zhang, V. Adivarahan, S. Wu, Grigory Simin, M. Asif Khan

Faculty Publications

We present a study of the electrical and optical characteristics of 285 nm emission deep ultraviolet light-emitting diodes(LED) at temperatures from 10 to 300 K. At low bias, our data show the tunneling carrier transport to be the dominant conduction mechanism. The room-temperature performance is shown to be limited mostly by poor electron confinement in the active region and a pronounced deep level assisted recombination but not by the hole injection into the active region. At temperatures below 100 K, the electroluminescence peak intensity increases by more than one order of magnitude indicating that with a proper device design and …

Near-Band-Edge Photoluminescence Of Wurtzite-Type Aln, E. Kuokstis, J. Zhang, Q. Fareed, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. Shur, C. Rojo, L. Schowalter

Faculty Publications

Temperature-dependentphotoluminescence(PL)measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electron–hole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electron–hole plasma expansion peculiarities.

Analysis Of Error Recovery Effects On Digital Flight Control Systems, Arturo Tejada Ruiz

Electrical & Computer Engineering Theses & Dissertations

Life-critical, real-time applications like flight-by-wire aircraft, rely on closed-loop digital control systems and fault-tolerant computer systems to reliably achieve the desired operation. The computer systems, however, may be affected by random hardware/software faults induced mainly by environmental conditions such as high intensity radiated fields (HIRF) and lightning. These harsh electromagnetic environments are known to induce common­ mode faults (CMF) in aircraft electronic systems, which disrupt fault-tolerant provisions and possibly affect the operation of the digital control system. Current flight-by-wire aircraft have computer systems that can neither detect CMF nor recover from them. New systems are under investigation that can recover …

Nonresonant Detection Of Terahertz Radiation In Field Effect Transistors, W. Knap, V. Kachorovskii, Y. Deng, S. Rumyantsev, J.-Q. Lü, R. Gaska, M. S. Shur, Grigory Simin, X. Hu, M. Asif Khan, C. A. Saylor, L. C. Brunel

Faculty Publications

We present an experimental and theoretical study of nonresonant detection of subterahertz radiation in GaAs/AlGaAs and GaN/AlGaN heterostructurefield effect transistors. The experiments were performed in a wide range of temperatures (8–300 K) and for frequencies ranging from 100 to 600 GHz. The photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage. The results were interpreted using a theoretical model that shows that the maximum in photoresponse can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.

Time-Resolved Photoluminescence Of Quaternary Alingan-Based Multiple Quantum Wells, Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan, G. G. Sim, P. W. Yu

Faculty Publications

Time-resolvedphotoluminescence(PL)dynamics has been studied in AlInGaN/AlInGaN multiple quantum wells(MQWs) grown by a pulsed metalorganic chemical vapor deposition (PMOCVD) procedure. The PL decay kinetics was found to be sensitive to the emission energy and temperature. The PL decay time increases with decreasing emission energy, which is a characteristic of localized carrier/exciton recombination due to alloy fluctuations. Its temperature dependence shows radiative recombination to be the dominant process at low temperatures, indicating a high quality of PMOCVD grown quaternary AlInGaN MQWs and establishing them as promising structures for the active region of deep ultraviolet light emitting diodes.

Luminescence Mechanisms In Quaternary AlXInYGa1-X-YN Materials, Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan

Faculty Publications

Low-temperature photoluminescence investigations have been carried out in the quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by pulsedmetalorganic chemical-vapor deposition (PMOCVD). With increasing excitation power density, the emission peaks in both AlInGaN epilayers and MQWs show a strong blueshift and theirlinewidths increase. The luminescence of the samples grown by PMOCVD is attributed to recombination of carriers/excitons localized at band-tail states. We also demonstrate theluminescence properties of AlInGaN and AlGaN materials grown by a pulsed atomic-layerepitaxy and conventional MOCVD, respectively.

Crack-Free Thick Algan Grown On Sapphire Using Aln/Algan Superlattices For Strain Management, J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, Grigory Simin, M. Asif Khan

Faculty Publications

We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n⁺-AlGaNlayers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al_0.2Ga_0.8N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates.

A Study Of Radiofrequency Effects On Gene Expression, Elizabeth A. Coln

Electrical & Computer Engineering Theses & Dissertations

Numerous studies have been completed over the past several years to look at the non-thermal effects of low-level radiofrequency exposure on biological systems. These studies were done to determine if exposure to radiofrequency fields are safe, or if they may lead to health problems or changes to any of the biological processes continually occurring within the human body. Responses measured include stress responses (I), gene expression (2), cell proliferation (3), cell morphology (4), and changes in DNA (5).

Dispersion is the most common model for explaining the electric behavior of biomaterials. It refers to the corresponding frequency domain concept of …

Two Mechanisms Of Blueshift Of Edge Emission In Ingan-Based Epilayers And Multiple Quantum Wells, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur

Faculty Publications

We present the results of a comparative photoluminescence(PL) study of GaN and InGaN-based epilayers, and InGaN/GaN multiple quantum wells(MQWs). Room-temperature PL spectra were measured for a very broad range of optical excitation from 10 mW/cm² up to 1 MW/cm². In contrast to GaN epilayers, all In-containing samples exhibited an excitation-induced blueshift of the peak emission. In addition, the blueshift of the emission in the InGaN epilayers with the same composition as the quantum well was significantly smaller. The comparison of the blueshift in the “bulk” InGaN and in the MQWs allowed us to separate two different mechanisms …

Implementation Of A Si/Sic Hybrid Optically Controlled High-Power Switching Device, Prashant Bhadri, Kuntao Ye, Elena A. Guliants, Fred R. Beyette Jr.

Electrical and Computer Engineering Faculty Publications

The ever-increasing performance and economy of operation requirements placed on commercial and military transport aircraft are resulting in very complex systems. As a result, the use of fiber optic component technology has lead to high data throughput, immunity to EMI, reduced certification and maintenance costs and reduced weight features. In particular, in avionic systems, data integrity and high data rates are necessary for stable flight control. Fly-by-Light systems that use optical signals to actuate the flight control surfaces of an aircraft have been suggested as a solution to the EMI problem in avionic systems. Current fly-by-light systems are limited by …

Translation Of 'Balika Badhu: A Selected Anthology Of Bengali Short Stories', Monish Ranjan Chatterjee

Electrical and Computer Engineering Faculty Publications

This project, which began with the desire to render into English a rather long tale by Bimal Kar about five years ago, eventually grew into a considerably more extended compilation of Bengali short stories by 10 of the most well-known practitioners of that art since the heyday of Rabindranath Tagore. The collection is limited in many ways, not the least of which being that no woman writer has been included, and that it contains only a baker's dozen stories (if we count Bonophool's micro-stories collectively as one ) — a number pitifully small considering the vast and prolific field of …

Effect Of Ga Content On Defect States In Cuin1-XGaXSe2 Photovoltaic Devices, Jennifer T. Heath, J. David Cohen, William N. Shafarman, Dongxiang Liao, Angus Rockett

Faculty Publications

Defects in the band gap of CuIn_1-xGa_xSe₂ have been characterized using transient photocapacitance spectroscopy. The measured spectra clearly show response from a band of defects centered around 0.8 eV from the valence band edge as well as an exponential distribution of band tail states. Despite Ga contents ranging from Ga/(In+Ga)=0.0 to 0.8, the defect bandwidth and its position relative to the valence band remain constant. This defect band may act as an important recombination center, contributing to the decrease in device efficiency with increasing Ga content.

The Effect Of Weak Acids Hydrolysis On The Sol-Gel Fabrication Of Silica Thin Film, Abdul Hamid

Student Works (2000-2009)

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Modelling Of Space Charge Effects In The Electrical Discharge Of Nitrogen Gas Laser, Chin Yang Chia

Student Works (2000-2009)

A computer model for the simulation of a nitrogen gas discharge has been built and studied. This model was based on the fluid description of the discharge plasma. The Phoenical SHASTA Flux-Corrected Transport (FCT) algorithm is used for the solution of the fluid equations and its accuracy has been tested. The distribution of the local electrical field was calculated by the Poisson equation. The internal discharge current was calculated by Sato's equation. The motion of the charge particles was taken into account. The electrical discharge behavior was simulated by the circuit equation and solved using the fourth order Runge-Kutta method …

A 0.5 Μm Thick Polysilicon Schottky Diode With Rectification Ratio Of 10^6, Elena A. Guliants, Chunhai Ji, Young J. Song, Wayne A. Anderson

Electrical and Computer Engineering Faculty Publications

Polycrystalline Si films, 0.5-mm thick, were obtained as a result of metal-induced growth by sputtering from a Si target on 25 nm thick Ni prelayers at 525 °C. Silicon grew heteroepitaxially on the NiSi2 layer formed due to the reaction between the sputtered Si atoms and Ni. Schottky diodes were fabricated on the Si films by deposition of a Schottky metal on the front surface of the film while Ni disilicide provided an intimate ohmic contact at the back. A Pd/n-Si diode using an n-Si film annealed for 2 h at 700 °C in forming gas demonstrated a rectification ratio …

Self-Assembly Of Spatially Separated Silicon Structures By Si Heteroepitaxy On Ni Disilicide, Elena A. Guliants, Chunhai Ji, Wayne A. Anderson

Electrical and Computer Engineering Faculty Publications

A nonlithographic approach to produce self-assembled spatially separated Si structures for nanoelectronic applications was developed, employing the metal-induced silicon growth. Densely packed Si whiskers, 500–800 nm thick and up to 2500 nm long, were obtained by magnetron sputtering of Si on a 25 nm thick Ni prelayer at 575 °C. The nucleation of the NiSi2 compound at the Ni–Si interface followed by the Si heteroepitaxy on the lattice-matched NiSi2 is suggested to be the driving force for the whisker formation.

Acceleration Element For Femtosecond Electron Pulse Compression, Bao-Liang Qian, Hani E. Elsayed-Ali

Electrical & Computer Engineering Faculty Publications

An acceleration element is proposed for compressing the electron pulse duration in a femtosecond photoelectron gun. The element is a compact metal cavity with curved-shaped walls. An external voltage is applied to the cavity where a special electric field forms in such a way that the slow electrons in the electron pulse front are accelerated more than the fast electrons, and consequently the electron pulse duration will be compressed. The distribution of the electric field inside the acceleration cavity is analyzed for the geometry of the cavity. The electron dynamics in this acceleration cavity is also investigated numerically. Numerical results …